The Coronary Drug Project (CDP) was undertaken to evaluate the efficacy of several different lipid-lowering drugs in the long-term treatment of coronary heart disease and to study the natural history and clinical course of this disease. The CDP is a multiclinic study consisting of 53 Clinical Centers, a Coordinating Center, a Central Laboratory, an ECG Reading Center, and a Drug Procurement and Distribution Center. A total of 8,341 patients were enrolled in the study during the recruitment period which lasted from March 1966 through October 1969. Only males between the ages of 30 and 64 having a history of at least on ECG-documented myocardial infarction, and belonging to MYHA functional class I or II were eligible for enrollment in the study. Each patient was randomly assigned to one of the following six treatment groups: Mixed conjugated equine estrogens (two dosage levels), clofibrate, dextrothyroxine, nicotinic acid, and placebo. All patients are to be followed for minimum of five years on their assigned CDP medication plus a final four months off their CDP medication unless definitive results allow termination before five years. The terminal four-month period will end in May 1975. The information obtained from this study is being used to evaluate the CDP drugs with respect to effects on mortality, cardiovascular morbidity, and serum lipids as well as side effects, biochemical effects, and electrocardiographic changes. Placebo group data on the demographic, clinical, electrocardiographic, biochemical, pharmacologic, and radiographic variable observed at baseline, well as other aspects of the natural history and clinical course of coronary heart disease, will be analyzed for prognostic significance. In one subgroup efficacy of aspirin compared with placebo will be examined with the expectation that a platelet inhibiting effect may have an influence on occurence or outcome of myocardial infarction.